For example, a fluid to be measured continuously flows from a liquid chromatograph and is introduced into a flow cell such that a characteristic of the fluid is inspected by a desirable detector.
FIG. 1 shows an example of such a flow cell.
In this drawing, a flow liquid 10 from a liquid chromatograph is introduced into a rectangular flow cell 12, which is made of quartz glass, from therebelow. Then, excitation light L1 from a light source 14 is made incident on one side surface 12a of the flow cell 12. When the flow liquid 10 to be measured within the flow cell 12 emits fluorescence, fluorescence L2 obtained in a direction orthogonal to the excitation light L1 is detected by a detector 16.
Thus configured flow cell 12 can continuously measure the fluorescent materials existing in the flow liquid 10 flowing from the liquid chromatograph.
Here, a tube having a bore of not greater than 0.25 mm is usually used for connecting a column and a flow cell to each other in order to prevent the sample separated in the column from diffusing within the tube. Accordingly, at a usual liquid supply flow rate of 0.5 to 3.0 ml/min, the average flow velocity within the tube becomes 17 to 100 cm/sec, which is very fast.
As the quartz cell used as a flow cell, one having a capacity of 10 to 50 .mu.l is used as a standard cell for a liquid chromatograph. Based on this capacity, the flow cell has a size within the range of about 1.5.times.1.5 (defining the size of a plane perpendicular to the flow direction).times.5.0 mm (defining the size in the flow direction) to 3.0.times.3.0.times.5.0 mm. Though the sensitivity of the detector increases as the capacity of the flow cell is larger, two components, for example, which are sufficiently separated from each other by a column may mix within the flow cell, thereby making it difficult for a chromatogram, which records the signal output from the detector, to sufficiently separate these two components from each other and thus deteriorating the accuracy in quantitative analysis. Therefore, the capacity of the flow cell should be restricted to the above-mentioned range.
As explained above, since the diameter of the tube connecting the column and the flow cell to each other and that of the flow cell greatly differ from each other, the flow cell and the tube have conventionally been connected to each other as shown in FIG. 2.
In FIG. 2, the flow cell 12 and a tube 18 is connected to each other by a flow-cell holding portion 20 and an inlet gasket 22. As shown in this drawing, an opening 20a is formed at the center of the flow-cell holding portion 20 so as to receive the tube 18, whereas the upper portion of the opening 20a is this drawing has an opening 20b having a slightly larger diameter into which the inlet gasket 22 is fitted. The tube 18 and inlet gasket 22 positioned by these openings 20a and 20b are fixed such that their conduits 18a and 22a communicate with each other.
On the other hand, the flow cell 12 is tightly fixed such that its lower opening 12b is pressed against the inlet gasket 22.
When a buffer solution is used as a mobile phase for a high-performance liquid chromatograph (HPLC), a salt in the buffer solution may be deposited on the inner surface of the tube connecting the column and the flow cell to each other, thereby blocking the tube or, when a fruid to be measured is supplied to thus blocked tubing from the flow cell portion, the flow cell may be broken. Also, when the apparatus is not used for a long time, mold or the like may be generated, thereby blocking the tube or breaking the flow cell as well.
When the tube is blocked, since it is relatively easy to obtain a new tube, not a skilled maintenance engineer but a user usually exchanges thus blocked tube for a new one. Also, when the flow cell is broken, not a skilled maintenance engineer but a user usually exchanges thus broken flow cell for a new one.
However, there has been a problem that the peak resolution remarkably decreases when the tube or flow cell is exchanged.
In order to investigate the cause of this problem, the inventors have conducted the following tests.
Namely, the following Table 1 (A, B group) shows the results of comparison of the case in which a special tool is used for adjusting the position where the flow cell is attached with the case in which the position where the flow cell is attached is adjusted by visual observation without using the special tool. Also, this table (C, D group) shows the results of comparison of the case in which burrs formed upon processing of the inlet end surface of the tube are sufficiently removed with the case in which they are not sufficiently removed.
In this analysis system, in order to verify the diffusion within the cell, acetonitrile as a mobile phase, 0.1% benzene as a sample, and a reversed-phase column are used so as to generate a condition under which the sample is not retained within the column.
(Compared Items) PA0 (Method of Evaluation)
A: Case in which a special tool is used for adjusting the position where the flow cell is attached.
B: Case in which, without using a special tool, the position where the flow cell is attached is adjusted by visual observation.
C: Case in which burrs formed upon processing of the inlet end surface of the tube are sufficiently removed.
D: Case in which burrs formed upon processing of the inlet end surface of the tube are not sufficiently removed.
In this analysis system, the number of theoretical plates (NTP) of benzene peak is used as an index for evaluation, which is represented by the following equations (1) and (2): ##EQU1## wherein TR is retention time of benzene peak, A is area value of benzene peak, and H is height value of benzene peak.
TABLE 1 ______________________________________ Compared Item Conventional Flow Cell ______________________________________ A group 5,600 B group 4,100 C group 5,450 D group 3,950 ______________________________________
For example, in FIG. 3, in order to exchange the tube 18 for a new one, an inlet end surface 18c having a bore of not greater than 0.25 mm has to be processed. The user who is not skilled in such processing cannot sufficiently remove the burrs formed upon the processing of the inlet end surface 18c and thus cannot uniformly process the flow path surface shape of the tube 18. Accordingly, as shown in Table 1, the processing accuracy at the inlet end surface 18c of the tube 18 cannot be controlled. When the flow path surface is not uniformly shaped, the flow rate distribution of the fluid running through the flow cell 12 may be disturbed.
Also, it is very difficult even for a skilled engineer to sufficiently remove the burrs formed when the inlet end surface 18c of the tube 18 having a bore of not greater than 0.25 mm is processed. When the processing accuracy of the inlet end surface 18c is increased to an extent where the flow rate distribution of the fluid running through the flow cell 12 is not disturbed, its manufacturing cost may increase, thereby making it impossible to reduce the cost.
Further, as mentioned above, the flow cell has a size within the range of about 1.5.times.1.5 (defining the size of a plane perpendicular to the flow direction).times.50 mm (defining the size in the flow direction) to 3.0.times.3.0.times.5.0 mm and is very small. Accordingly, in order to exchange the flow cells, the position where the flow cell is attached has to be adjusted within the range of 1 mm or less. Such an adjustment is very difficult for a user who has neither special tool nor skill. As shown in Table 1 (A, B Group), when the position where the flow cell is attached is not adjusted appropriately, the flow rate distribution of the fluid running through the flow cell may be disturbed as well.
Thus, when the user performs maintenance of the apparatus, the flow rate distribution of the fluid running through the flow cell may be disturbed in any case. Then, when the flow rate distribution of the fluid running through the flow cell is deteriorated, the two components which have been sufficiently separated from each other by the column may mix within the flow cell as mentioned above, thereby making it difficult for a chromatogram, which records the signal output from the detector, to sufficiently separate these two components from each other and thus deteriorating the accuracy in quantitative analysis.
In order to overcome these problems, a skilled maintenance engineer may be asked to make repairs. However, as compared with the case where the user performs maintenance, it takes days for the skilled engineer to come while necessitating labor cost to be paid therefor. Accordingly, it is not a preferable means for overcoming the problems.